CHAPTER CHAPTER 1313 – The – The CnidariansCnidarians

(Radiate Animals)(Radiate Animals)

• 13.1. Position and 13.1. Position and ContributionsContributions

•A. Position in Animal A. Position in Animal Kingdom Kingdom

– 1. Both phyla Cnidaria 1. Both phyla Cnidaria and Ctenophora make and Ctenophora make up the radiate animals. up the radiate animals.

– 2. Other eumetazoans 2. Other eumetazoans have bilateral have bilateral symmetry or their symmetry or their radial symmetry is radial symmetry is derived from a derived from a bilateral ancestor. bilateral ancestor.

– 3. 3. NeitherNeither Cnidaria nor Cnidaria nor Ctenophora Ctenophora have have advanced beyond advanced beyond tissue level of tissue level of organizationorganization although although a few organs are seen. a few organs are seen.

• B. Biological ContributionsB. Biological Contributions

– 1. Both phyla have 1. Both phyla have two well-defined two well-defined germ layersgerm layers: ectoderm and endoderm; : ectoderm and endoderm; mesoderm may be derived from mesoderm may be derived from ectoderm. ectoderm.

– 2. There is an internal body cavity: the 2. There is an internal body cavity: the gastrovascular cavitygastrovascular cavity. .

• B. Biological ContributionsB. Biological Contributions – 1. Both phyla have 1. Both phyla have two well-defined germ layerstwo well-defined germ layers: ectoderm : ectoderm

and endoderm; mesoderm may be derived from ectoderm. and endoderm; mesoderm may be derived from ectoderm.

– 2. There is an internal body cavity: the 2. There is an internal body cavity: the gastrovascular gastrovascular cavitycavity. .

– 3. 3. Extracellular digestionExtracellular digestion occurs in the gastrovascular occurs in the gastrovascular cavity; gastrodermal cells accomplish cellular digestion. cavity; gastrodermal cells accomplish cellular digestion.

• B. Biological ContributionsB. Biological Contributions

– 1. Both phyla have 1. Both phyla have two well-defined two well-defined germ layersgerm layers: ectoderm and : ectoderm and endoderm; mesoderm may be endoderm; mesoderm may be derived from ectoderm. derived from ectoderm.

– 2. There is an internal body cavity: 2. There is an internal body cavity: the the gastrovascular cavitygastrovascular cavity. .

– 3. 3. Extracellular digestionExtracellular digestion occurs occurs in the gastrovascular cavity; in the gastrovascular cavity; gastrodermal cells accomplish gastrodermal cells accomplish cellular digestion. cellular digestion.

– 4. Most have tentacles, which are 4. Most have tentacles, which are extensible projections for food extensible projections for food capture. capture.

– 5. Radiates are the simplest animals 5. Radiates are the simplest animals with with nerve cellsnerve cells; there is ; there is no central no central nervous systemnervous system. .

– 6. Radiates are the simplest animals 6. Radiates are the simplest animals with sense organs: statocysts and with sense organs: statocysts and ocelli. ocelli.

• B. Biological ContributionsB. Biological Contributions

– 1. Both phyla have 1. Both phyla have two well-defined germ two well-defined germ layerslayers: ectoderm and endoderm; : ectoderm and endoderm; mesoderm may be derived from mesoderm may be derived from ectoderm. ectoderm.

– 2. There is an internal body cavity: the 2. There is an internal body cavity: the gastrovascular cavitygastrovascular cavity. .

– 3. 3. Extracellular digestionExtracellular digestion occurs in the occurs in the gastrovascular cavity; gastrodermal cells gastrovascular cavity; gastrodermal cells accomplish cellular digestion. accomplish cellular digestion.

– 4. Most have tentacles, which are 4. Most have tentacles, which are extensible projections for food capture. extensible projections for food capture.

– 5. Radiates are the simplest animals with 5. Radiates are the simplest animals with nerve cellsnerve cells; there is ; there is no central nervous no central nervous systemsystem. .

– 6. Radiates are the simplest animals with 6. Radiates are the simplest animals with sense organs: statocysts and ocelli. sense organs: statocysts and ocelli.

– 7. Locomotion is by muscular contraction 7. Locomotion is by muscular contraction or ciliary comb plates. or ciliary comb plates.

– 8. The polyp and medusa forms allow 8. The polyp and medusa forms allow wider ecological possibilities. wider ecological possibilities.

– 9. Unique features include nematocysts, 9. Unique features include nematocysts, colloblasts and ciliary comb plates. colloblasts and ciliary comb plates.

• 13.2. Phylum Cnidaria13.2. Phylum Cnidaria • A. Cnidarian Life History A. Cnidarian Life History

– 1. Over 9,000 species are in the 1. Over 9,000 species are in the phylum phylum CnidariaCnidaria. .

– 2. Cnidaria have specialized cells 2. Cnidaria have specialized cells ((cnidocytescnidocytes) that contain a ) that contain a specialized stinging organelle, specialized stinging organelle, the the nematocystnematocyst. .

– 3. 3. Nematocysts are only formed Nematocysts are only formed and used by Cnidariansand used by Cnidarians. .

– 4. Cnidarians originated close to 4. Cnidarians originated close to the base of the metazoan the base of the metazoan lineage. lineage.

– 5. Today, they are most common 5. Today, they are most common in shallow marine environments, in shallow marine environments, some are freshwater but none some are freshwater but none are terrestrial. are terrestrial.

– 6. Some ctenophores, molluscs 6. Some ctenophores, molluscs and flatworms eat hydroids and and flatworms eat hydroids and use the stinging nematocysts in use the stinging nematocysts in their own defense. their own defense.

– 7. Some live symbiotically; algae 7. Some live symbiotically; algae in reef-building corals are critical in reef-building corals are critical to coral reef formation. to coral reef formation.

• B. Characteristics of B. Characteristics of CnidariaCnidaria

– 1. All are aquatic and 1. All are aquatic and mostly marine. mostly marine.

– 2. Radial or biradial 2. Radial or biradial symmetry forms oral and symmetry forms oral and aboral ends. aboral ends.

– 3. The two body types are 3. The two body types are the free-swimming the free-swimming medusaemedusae and the and the polypspolyps. .

– 4. Some have an 4. Some have an exoskeleton or exoskeleton or endoskeleton of endoskeleton of chitinous, calcareous or chitinous, calcareous or protein components. protein components.

• B. Characteristics of CnidariaB. Characteristics of Cnidaria

– 1. All are aquatic and mostly marine. 1. All are aquatic and mostly marine.

– 2. Radial or biradial symmetry forms oral and 2. Radial or biradial symmetry forms oral and aboral ends. aboral ends.

– 3. The two body types are the free-swimming 3. The two body types are the free-swimming medusaemedusae and the and the polypspolyps. .

– 4. Some have an exoskeleton or endoskeleton of 4. Some have an exoskeleton or endoskeleton of chitinous, calcareous or protein components. chitinous, calcareous or protein components.

– 5. They have a diploblastic body, with two layers: 5. They have a diploblastic body, with two layers: epidermis and gastrodermis; some are triploblastic epidermis and gastrodermis; some are triploblastic with an with an ectomesodermectomesoderm. .

– 6. The gastrovascular cavity has a 6. The gastrovascular cavity has a single openingsingle opening serving as both mouth and anus. serving as both mouth and anus.

– 7. Special stinging cells called nematocysts are in 7. Special stinging cells called nematocysts are in epidermis or gastrodermis and abundant on epidermis or gastrodermis and abundant on tentacles. tentacles.

– 8. The nerve net may include some sensory 8. The nerve net may include some sensory organs. organs.

– 9. The muscular system has an outer layer of 9. The muscular system has an outer layer of longitudinal fibers and an inner layer of circular longitudinal fibers and an inner layer of circular fibers. fibers.

– 10. Reproduction is either asexual or sexual; a 10. Reproduction is either asexual or sexual; a planula larva may be present. planula larva may be present.

– 11. There is 11. There is no excretory or respiratory systemno excretory or respiratory system. .

– 12. There is 12. There is no coelomic cavityno coelomic cavity. .

• B. Characteristics of CnidariaB. Characteristics of Cnidaria

– 1. All are aquatic and mostly marine. 1. All are aquatic and mostly marine.

– 2. Radial or biradial symmetry forms oral and 2. Radial or biradial symmetry forms oral and aboral ends. aboral ends.

– 3. The two body types are the free-swimming 3. The two body types are the free-swimming medusaemedusae and the and the polypspolyps. .

– 4. Some have an exoskeleton or endoskeleton of 4. Some have an exoskeleton or endoskeleton of chitinous, calcareous or protein components. chitinous, calcareous or protein components.

– 5. They have a diploblastic body, with two layers: 5. They have a diploblastic body, with two layers: epidermis and gastrodermis; some are triploblastic epidermis and gastrodermis; some are triploblastic with an with an ectomesodermectomesoderm. .

– 6. The gastrovascular cavity has a 6. The gastrovascular cavity has a single openingsingle opening serving as both mouth and anus. serving as both mouth and anus.

– 7. Special stinging cells called nematocysts are in 7. Special stinging cells called nematocysts are in epidermis or gastrodermis and abundant on epidermis or gastrodermis and abundant on tentacles. tentacles.

– 8. The nerve net may include some sensory 8. The nerve net may include some sensory organs. organs.

– 9. The muscular system has an outer layer of 9. The muscular system has an outer layer of longitudinal fibers and an inner layer of circular longitudinal fibers and an inner layer of circular fibers. fibers.

– 10. Reproduction is either asexual or sexual; a 10. Reproduction is either asexual or sexual; a planula larva may be present. planula larva may be present.

– 11. There is 11. There is no excretory or respiratory systemno excretory or respiratory system. .

– 12. There is 12. There is no coelomic cavityno coelomic cavity. .

• B. Characteristics of B. Characteristics of CnidariaCnidaria

– 12. There is 12. There is no coelomic no coelomic cavitycavity. .

• C. Form and Function C. Form and Function

– 1. Cnidaria have two basic body 1. Cnidaria have two basic body plans. plans.

– 2. A 2. A polyp polyp is a hydroid form. is a hydroid form.

» a. Polyps are an adaptation to a. Polyps are an adaptation to a sedentary life. a sedentary life.

» b. The body is tubular with the b. The body is tubular with the mouth directed upward and mouth directed upward and surrounded by tentacles. surrounded by tentacles.

» c. The aboral end is attached c. The aboral end is attached to a substrate by a pedal disc. to a substrate by a pedal disc.

» d. In colonial forms, the polyps d. In colonial forms, the polyps may be specialized for may be specialized for feeding, reproduction or feeding, reproduction or defense. defense.

» e. Sea anemones and corals e. Sea anemones and corals are all polyps with no medusa are all polyps with no medusa stage. stage.

• 3. A 3. A medusa medusa is bell or umbrella-is bell or umbrella-shaped. shaped.

– a. The mouth is directed a. The mouth is directed downward; tentacles may downward; tentacles may extend down from the rim of the extend down from the rim of the umbrella. umbrella.

– b. In tetramerous symmetry, b. In tetramerous symmetry, body parts are arranged in body parts are arranged in fours. fours.

– c. Medusae dominate the life of c. Medusae dominate the life of true jellyfish, but many have a true jellyfish, but many have a polypoid form at the same time. polypoid form at the same time.

• 4. Hydrozoans often have life 4. Hydrozoans often have life histories featuring both stages. histories featuring both stages.

• 5. Both stages have retained the 5. Both stages have retained the sac-like body plan typical of the sac-like body plan typical of the phylum. phylum.

• 6. Both have three body layers; 6. Both have three body layers; medusa has a much thicker medusa has a much thicker mesoglea than do polyps. mesoglea than do polyps.

• D. Nematocysts: Stinging Organelles D. Nematocysts: Stinging Organelles

– 1. Over 20 different types of nematocysts have been described; they 1. Over 20 different types of nematocysts have been described; they are important in taxonomy. are important in taxonomy.

– 2. 2. Nematocysts Nematocysts are tiny capsules made of are tiny capsules made of chitin-like materialchitin-like material and and containing a coiled filament. containing a coiled filament.

– 3. A little lid or 3. A little lid or operculum operculum covers the end of the capsule. covers the end of the capsule.

– 4. The inside of the thread may have tiny barbs or spines. 4. The inside of the thread may have tiny barbs or spines.

– 5. The cnidocyte is the cell that produces the nematocyst; it develops 5. The cnidocyte is the cell that produces the nematocyst; it develops from a from a cnidoblastcnidoblast. .

– 6. Except in Anthozoa, a modified cilium called a 6. Except in Anthozoa, a modified cilium called a cnidocil cnidocil functions as functions as a trigger. a trigger.

– 7. Both small organic molecules and vibrations sensitize anthozoan 7. Both small organic molecules and vibrations sensitize anthozoan cnidocytes. cnidocytes.

– 8. After a nematocyte is discharged, its cnidocyte is absorbed and 8. After a nematocyte is discharged, its cnidocyte is absorbed and another develops. another develops.

– 9. Some lack barbs or poison and rapidly recoil, grasping and holding 9. Some lack barbs or poison and rapidly recoil, grasping and holding prey. prey.

• 10. 10. Mechanism of Nematocyst Mechanism of Nematocyst DischargeDischarge

– a. The cell can generate a high a. The cell can generate a high osmotic pressure of osmotic pressure of 140 140 atmospheresatmospheres within the within the cnidocyte. cnidocyte.

– b. The osmotic pressure falls as b. The osmotic pressure falls as the hydrostatic pressure the hydrostatic pressure increases. increases.

– c. When stimulated, the high c. When stimulated, the high internal osmotic pressure causes internal osmotic pressure causes water to rush into the capsule. water to rush into the capsule.

– d. The operculum opens and d. The operculum opens and rapidly releases the increased rapidly releases the increased hydrostatic pressure, launching hydrostatic pressure, launching the thread. the thread.

– e. At the everting end of the e. At the everting end of the thread, the barbs point backward thread, the barbs point backward to anchor. to anchor.

– f. Poison may be injected when it f. Poison may be injected when it penetrates the prey. penetrates the prey.

• 11. Only a few jellyfish and the 11. Only a few jellyfish and the Portuguese man-of-war can seriously Portuguese man-of-war can seriously harm humans. harm humans.

• E. E. Nerve Net Nerve Net

– 1. Two nerve nets, one at the 1. Two nerve nets, one at the base of epidermis and one at the base of epidermis and one at the base of gastrodermis, base of gastrodermis, interconnect. interconnect.

– 2. Nerve impulses move across 2. Nerve impulses move across synapsessynapses by neurotransmitters. by neurotransmitters.

– 3. 3. Unlike higher animals, Unlike higher animals, cnidarian nerves have cnidarian nerves have neurotransmitters on both sides neurotransmitters on both sides of the synapses allowing of the synapses allowing transmission in either directiontransmission in either direction. .

– 4. Cnidarian nerves lack the 4. Cnidarian nerves lack the myelin sheath on axons. myelin sheath on axons.

– 5. There is no central nervous 5. There is no central nervous system, but ring nerves serve as system, but ring nerves serve as marginal sense organs of marginal sense organs of medusae. medusae.

– 6. In some species, a fast 6. In some species, a fast network coordinates swimming network coordinates swimming movements, a slower one movements, a slower one coordinates tentacles. coordinates tentacles.

– 7. 7. This nerve net pattern is also This nerve net pattern is also found in the digestive systems of found in the digestive systems of annelids (earthworms), humans annelids (earthworms), humans and other organismsand other organisms. .

• F. F. Class Hydrozoa Class Hydrozoa

– 1. Hydra: A Freshwater Hydrozoan (Fig. 13-6) 1. Hydra: A Freshwater Hydrozoan (Fig. 13-6)

» a. a. HydraHydra are solitary polyps and one of a are solitary polyps and one of a few cnidarians found in freshwater. few cnidarians found in freshwater.

» b. The body is a cylindrical tube; the aboral b. The body is a cylindrical tube; the aboral end has a basal or pedal disc for end has a basal or pedal disc for attachment. attachment.

» c. A ring of hollow tentacles encircles the c. A ring of hollow tentacles encircles the mouth. mouth.

» d. The mouth opens to a gastrovascular d. The mouth opens to a gastrovascular cavity. cavity.

» e. e. UndifferentiatedUndifferentiated interstitial cellsinterstitial cells can develop into cnidoblasts, sex cells, can develop into cnidoblasts, sex cells, buds, or nerve cells, but not buds, or nerve cells, but not epitheliomuscular cells. epitheliomuscular cells.

» f. Gland cells on the adhesive disc secrete f. Gland cells on the adhesive disc secrete an adhesive and sometimes a gas bubble an adhesive and sometimes a gas bubble for floating. for floating.

» g. Hydras have nematocysts that: 1) g. Hydras have nematocysts that: 1) penetrate prey and inject poison; 2) recoil penetrate prey and inject poison; 2) recoil and entangle prey; and 3) secrete an and entangle prey; and 3) secrete an adhesive for locomotion and attachment. adhesive for locomotion and attachment.

• G. Class Scyphozoa G. Class Scyphozoa

– 1. Most of the larger 1. Most of the larger jellyfishesjellyfishes belong to belong to this class. this class.

– 2. Nearly all float in open sea. 2. Nearly all float in open sea.

– 3. Bells vary in shape and size; it is mostly 3. Bells vary in shape and size; it is mostly mesogleal jelly, which is 95-96% water. mesogleal jelly, which is 95-96% water.

– 4. Unlike hydromedusae, the mesoglea 4. Unlike hydromedusae, the mesoglea contains contains ameboid cellsameboid cells and fibers. and fibers.

– 5. The mouth is beneath the umbrella. 5. The mouth is beneath the umbrella.

– 6. A manubrium forms four oral arms to 6. A manubrium forms four oral arms to capture and ingest prey. capture and ingest prey.

– 7. Tentacles, manubrium, members of and 7. Tentacles, manubrium, members of and the entire body may have nematocysts. the entire body may have nematocysts.

– 8. The nervous system consists of a nerve 8. The nervous system consists of a nerve net; a subumbrellar net controls bell net; a subumbrellar net controls bell pulsations and a more diffuse net controls pulsations and a more diffuse net controls local reactions and feeding. local reactions and feeding.

– 9. Sexes are separate and 9. Sexes are separate and fertilization is fertilization is internalinternal in the gastric pouch of the in the gastric pouch of the female. female.

– 10. A zygote develops into a ciliated 10. A zygote develops into a ciliated planula larva; this attaches and develops planula larva; this attaches and develops into a into a scyphistomascyphistoma. .

• H. Class Anthozoa (Fig. 13-21) H. Class Anthozoa (Fig. 13-21)

• 1. 1. Sea AnemonesSea Anemones

– a. They attach to shells, rocks, timber, etc. by pedal discs; a. They attach to shells, rocks, timber, etc. by pedal discs; some burrow in mud or sand. some burrow in mud or sand.

– b. A crown of tentacles surrounds the flat oral disc. b. A crown of tentacles surrounds the flat oral disc.

– c. A slit-shaped mouth leads into a pharynx. c. A slit-shaped mouth leads into a pharynx.

– d. The d. The siphonoglyph siphonoglyph is a ciliated groove that creates the is a ciliated groove that creates the water current into the pharynx. water current into the pharynx.

– e. Currents carry in oxygen and remove wastes, and e. Currents carry in oxygen and remove wastes, and maintain fluid pressure for a hydrostatic skeleton. maintain fluid pressure for a hydrostatic skeleton.

– f. When in danger, water is rapidly expelled through pores as f. When in danger, water is rapidly expelled through pores as the anemone contracts to a small size. the anemone contracts to a small size.

– g. Most anemones can glide slowly on pedal discs; some can g. Most anemones can glide slowly on pedal discs; some can swim with limited ability. swim with limited ability.

– h. Escape reactions occur in response to extracts from h. Escape reactions occur in response to extracts from predators (e.g. sea stars, nudibranchs). predators (e.g. sea stars, nudibranchs).

– i. Most harbor symbiotic algae; some have a mutualistic i. Most harbor symbiotic algae; some have a mutualistic relationship with hermit crabs. relationship with hermit crabs.

– j. Some damselfishes shelter in sea anemones and have skin j. Some damselfishes shelter in sea anemones and have skin mucus that protects them from triggering nematocysts. mucus that protects them from triggering nematocysts.

• 2. Zoantharian Corals 2. Zoantharian Corals

– a. Members of the order Scleractinia are also known as a. Members of the order Scleractinia are also known as true or stony coralstrue or stony corals. .

– b. They are miniature sea anemones that live in calcareous cups they have b. They are miniature sea anemones that live in calcareous cups they have secreted. (Fig. 13-26) secreted. (Fig. 13-26)

– c. Instead of a pedal disc, they secrete a limey skeletal cup with sclerosepta c. Instead of a pedal disc, they secrete a limey skeletal cup with sclerosepta projecting up into the polyp. projecting up into the polyp.

– d. A sheet of living tissue forms over the coral surface, connecting all d. A sheet of living tissue forms over the coral surface, connecting all gastrovascular cavities. (Fig. 13-28) gastrovascular cavities. (Fig. 13-28)

• 3. 3. Coral ReefsCoral Reefs

– a. Coral reefs have great productivity, rivaled only by tropical rainforests. a. Coral reefs have great productivity, rivaled only by tropical rainforests.

– b. Living plants and animals are limited to the top layer above the calcium b. Living plants and animals are limited to the top layer above the calcium carbonate deposits. carbonate deposits.

– c. c. Hermatypic corals Hermatypic corals and and coralline algae coralline algae form most coral reefs. (Fig. 13-form most coral reefs. (Fig. 13-28) 28)

– d. These corals require full salinity of seawater and warmth and light, d. These corals require full salinity of seawater and warmth and light, limiting them to waters between 30 degrees north and south. limiting them to waters between 30 degrees north and south.

– e. Microscopic zooanthellae are photosynthetic and begin the food chain and e. Microscopic zooanthellae are photosynthetic and begin the food chain and recycle phosphorus and wastes. recycle phosphorus and wastes.

– f. These habitats support a diversity of corals and fish. f. These habitats support a diversity of corals and fish.

– g. Few nutrients enter or leave the system. g. Few nutrients enter or leave the system.

– h. Nutrients from fertilizer and sewage threaten coral reefs with excessive h. Nutrients from fertilizer and sewage threaten coral reefs with excessive algal growth. algal growth.

Fig. 13.1a